1. Field of the Invention
The present invention relates to a non-magnetic substrate used for a magnetic recording medium to be mounted in various magnetic recording devices such as the external memories of computers. The present invention also relates to a manufacturing method for the substrate, and a magnetic recording medium incorporating the substrate.
2. Prior Art
A substrate for a magnetic recording medium (hereinafter simply referred to as a medium), for high recording density, requires highly accurate surface roughness and mechanical properties. These properties are attained by highly developed precision processing and polishing. Demand for a substrate with higher accuracy surface roughness and mechanical properties comes from the necessity for a magnetic head to fly at a lower flying height over the magnetic medium. As the recording density increases, the flying height must be further reduced.
Aluminum alloy, etc. has chiefly been employed for the substrate for the medium. A circular non-magnetic metal substrate is formed by electroless plating a nickel-phosphorus layer on the aluminum substrate. The nickel-phosphorus layer is lapped and polished to a smooth mirror-finished surface. In addition the nickel-phosphorus layer is textured to prevent the magnetic head from sticking to the layer and to improve the magnetic recording characteristics of the medium. The polishing surface platen generally stacks a pair of surface platens. Pad are attached to the surfaces where the upper and lower surface platens contact each other. The substrate is fixed between the upper and lower pads with a carrier.
An abrasive suspension, such as slurry, is added between the upper and lower pads while revolving the surface platens. The sides of the substrates are polished to form mirror-finished sides. The entire area of the substrate side comes in contact with the pad acting as a matrix at a time. The texturing surface platen, in general, uses a matrix such as an abrasive tape and a pad cut in a roll, etc. running in one direction. A rubber roller presses the matrix into contact with a part of the side of a revolving substrate in such a manner that a longitudinal direction of the contact side lies at right angles to the direction of a running matrix. The addition of drops of the abrasive slurry suspension to the contact side forms a texture in circumferential direction of the substrate. The ratio of the contact side area to the total substrate side area is four percent or less.
A highly rigid, non-metal substrate has come to be used as a circular, non-magnetic substrate of late. The substrate consists, for instance, of glass. The desired high revolution speed of a substrate gives rise to substrate oscillation, etc. The high rigidity, however, tends to suppress substrate oscillation. The glass substrate includes a chemically strengthened substrate and a crystallized substrate. A chemically strengthened substrate is formed by press-forming glass powder into a circular glass plate or cutting a circular glass plate out of float-process plate glass, lapping and polishing surface of the circular glass plate, and chemically strengthening the glass plate. A crystallized substrate is formed by crystallizing a press-formed circular glass plate, and lapping and polishing surface of the circular glass plate. Lapping and polishing surface of the circular glass plate is conducted in the same way as a non-magnetic metal substrate made of an aluminum alloy.
Manufacturing a chemically strengthened glass substrate requires lapping and polishing followed by chemical strengthening. Polishing in the manufacture of a chemically strengthened glass substrate poses the problem that the polishing pad clogs up and deteriorates the same as in the manufacture of a non-magnetic metal substrate. In addition, chemical strengthening following the polishing process leaves occasional surface irregularities (irregular projections). To deal with this problem, additional polishing is required to remove the surface irregularities after chemical strengthening is completed. Polishing following chemical strengthening, however, leaves an alkaline residue component, which chemical strengthening has diffused into the surface through ion exchange. The alkaline residue tends to leach out of the strained layer, thereby degrading the mechanical properties of the substrate.
The present invention was achieved in view of the problems mentioned earlier.
Recent years have seen a requirement for increased recording density and reduced manufacturing costs of a magnetic recording apparatus. Increasing the recording density of a magnetic recording medium requires increased smoothness of a side of a non-magnetic substrate incorporated in a medium. The manufacturing surface platen for an aluminum alloy non-magnetic metal substrate comprises the following steps: lapping an aluminum alloy plate to smooth the side thereof, polishing several times (usually twice) the aluminum alloy plate to give a mirror-finished side, and texturing the aluminum alloy plate in the circumferential direction of the side.
Polishing processes the entire side of a substrate at a time. Polishing, on one hand, has the advantage of requiring a low contact pressure of a matrix. On the other hand, polishing has the disadvantage that it causes deterioration and clogging of the pad during processing. This makes it difficult to continuously produce uniform smoothness on the sides of the substrates. To cope with clogging, it has conventionally been required to stop processing on a regularly basis to clean the pad. This cleaning process is known as dressing. Cleaning the pad, however, does not eliminate deterioration of the pad. Thus, the pad must be exchanged for a new pad at a certain frequency. The higher the demand for surface roughness accuracy of a substrate, the more frequently is the need for cleaning and exchange of a pad. As a consequence, manufacturing cost is increased. Polishing cannot form suitable grooves in the circumferential direction of the substrate corresponding to those produced in a texturing process. In contrast to polishing, texturing, on one hand, eliminates the deterioration and clogging of a matrix thanks to the application of a constantly renewed matrix such as a pad to a substrate. Texturing, on the other hand, puts a matrix into contact with no more than some 4 percent or less of the area of the substrate side. Polishing, in contrast, processes the entire side of a substrate. Therefore, texturing is unsuitable for removing surface defects such as pits and nodules, etc. generated by plating. Texturing, further, has the problem that pressure per unit area applied to the substrate cannot be reduced. Increasing pressure per unit area applied to a substrate causes grit in the abrasive slurry increase its abrasive activity. This increases the difficulty in forming a smooth side due to roughening of the surface and generation of blemishes such as scratches, etc. Texturing is suitable for forming a texture, consisting of grooves in the circumferential direction of the substrate. However, as described above, texturing is no substitute for polishing in terms of forming a smooth and mirror finished side. Thus, manufacturing a non-magnetic metal substrate requires both polishing and texturing.
It is an object of the present invention to provide a non-magnetic substrate with increased surface roughness accuracy and improved machine characteristics.
Another object of the present invention is to provide a process for manufacturing a substrate with reduced manufacturing cost.
A further object of the present invention is to provide a magnetic recording medium with a high recording density using a substrate made as set forth earlier.
Briefly stated, the present invention provides a method in which a slowly advancing tape-shaped matrix is pressed into contact with a surface of a rapidly rotating non-magnetic substrate for a magnetic recording medium. An abrasive slurry is added to the matrix. The area contact rate between the matrix and the surface, as well as the contact pressure per unit area between the matrix and the surface are regulated to both polish and slightly roughen the surface in a single process. In the case of a glass presubstrate, the glass is chemically strengthened before being surface-finished.
According to an embodiment of the invention, there is provided a manufacturing process for a non-magnetic substrate for a magnetic recording medium comprising: pressing a running matrix of a surface-treating machine into contact with part of a side of a revolving presubstrate, adding an abrasive suspension to said matrix, and the step of pressing including pressing a contact rates of 15 to 25 percent by area of said side and at a pressure of 20 to 150 g/cm2 
According to a feature of the invention, there is provided a manufacturing process for a non-magnetic substrate for a magnetic recording medium comprising the step of: pressing a running matrix of a surface-treating machine into contact with part of a side of a revolving presubstrate, adding an abrasive suspension to said matrix, the step of pressing including contacting said substrate at contact rates of from about 2 to about 15 percent by area of said side, and the step of pressing further including pressing with a pressures of from about 25 to about to 800 g/cm2.